A variety of potential biological sources exist that can provide hydrocarbon molecules with chain lengths that are roughly appropriate for conversion into a diesel fuel. These biological sources can include vegetable fats or oils, animal fats or oils (including fish oils), or even fats or oils derived from algae. Based on regulatory activity by various governments, fuels derived from such biocomponent sources are likely to be increasingly important in the future.
Unfortunately, processing of biocomponent materials in conventional hydroprocessing equipment can be expensive from a refinery perspective. In particular, published literature reports of hydrogen consumption of biocomponent fuels during hydroprocessing indicate hydrogen needs in excess of 1000 scf/bbl (170 Nm3/m3). In addition to requiring large amounts of hydrogen, hydroprocessing of a biocomponent feed typically leads to production of CO and CO2. These contaminant species can be pose problems for conventional hydrogen scrubbing systems, making it difficult to recycle the excess hydrogen used for processing the biocomponent feed.
International Publication No. WO 2010/002903 describes a multi-stages hydroprocessing process and apparatus. In the process, a fresh feed is divided into a series of portions. All of the hydrogen for processing the feed is introduced into a first reactor stage. Additional portions of feed are introduced into subsequent reactors. The initial reactor stages are described as having a continuous gas phase environment. Based on the addition of additional feed in subsequent stages, it is described that the final stage(s) have a continuous liquid phase environment. Optionally, a portion of the liquid product can be recycled and combined with the portion of the fresh feed entering the first reaction stage.
U.S. Published Patent Application No. 2009/0095653 describes a hydroisomerization process. The hydroisomerization is performed in a reactor that has a substantially continuous liquid phase. An excess of hydrogen gas can be present beyond the solubility limit of the feedstock. However, the flowing medium in the reactor is described as being substantially liquid-continuous. The excess hydrogen gas is described as allowing the liquid phase to remain saturated with hydrogen as the reaction proceeds. The hydrocarbon feed is described as being a Fischer-Tropsch feed or a hydroprocessed vegetable oil composed primarily of n-paraffins in the C8 to C30 carbon number range.
U.S. Pat. No. 7,291,257 describes a system and method for two phase hydroprocessing. The method is described as allowing for hydroprocessing where the need to circulate hydrogen gas or a separate hydrogen phase through the catalyst is eliminated. Instead, the hydrogen for the hydroprocessing is dissolved in the feed, which can include a diluent to increase the capability of the feed for dissolving hydrogen. Optionally, additional amounts of hydrogen gas may be present of about 10% or less relative to the total volume of the reactor.